I’ve been pretty bullish on the Nexus-3 IGH for fat bike builds the last couple of months. The Nexus-3 IGH only adds a little over a pound to the weight of your ebike, but it drastically increases the usable range of your BBS02 or BBSHD motor over a single speed setup. It seems to be able to easily take 1500W of power, even when shifted under severe loads as well as being able to handle higher speeds of over 25mph without a problem. The only disadvantage of the Nexus 3 is that it will only fit on 135mm dropouts (like my Sunkiss and Duh Banana Bike builds) which are as rare in the fat bike world as an albino alligator. Luckily, Sturmey Archer built and is selling a 3-speed IGH that is specially designed for fat bikes called the SX-RK3 (their website information page is
Luckily, Sturmey Archer built and is selling a 3-speed IGH that is specially designed for fat bikes called the SX-RK3 (their website information page is here) which fits perfectly in 170mm rear dropouts and only weighs in at 1400g without the cog or shifting mechanism. This means that you can install a 3-speed IGH on most 4″ tire fat bikes on the market and only add a little over 2 lbs to the total weight of your fat bike (most decent rear hubs are about a pound). The SX-RK3 hub is available from bikeparts.com for about $95 here but you will also need some other mounting hardware and a shifter to make it work.
I tested a Sturmey Archer 3 speed IGH on my Burning Chrome build which is essentially a polished Soul Stomper with a 3000W Cyclone kit installed on it (installation instructions for the Cyclone are located here). I did not have to lace up the IGH to a fatbike rim which was a nice change as it came installed on the bike already. Getting the IGH set properly was a little tricky but there is an easy video on youtube located here that Jim who owns Soul Bikes put together. Like most bicycle videos on the internet (Bruno excluded) this video is terribly boring and SPOILER ALERT Darth Vader is Luke’s father, I know big surprise. What you already knew that? Worst kept secret in the galaxy.
Although the Sturmey Archer IGH with lots of gears seem to have mixed reviews, for the most part the 3-speed SA IGH has been quite bulletproof through the years. I’ve read lots of stuff on the internet from people who had the SA 8-speed IGH units self-destruct on them with normal biking activity and any powered e-bike is encouraged to use an Alfine 8-speed IGH (~$200 street price) if you want more than 3 gears. There is also the ridiculously expensive Rohloff Speedhub XL 14 gear IGH for fat bikes, but that runs around $1400 which is more than I pay to build most of my e-bikes with a battery and motor. I have never tested the Alfine or the Rohloff so I can’t speak to either one, but buy either one and ship it to me and I’ll beat the crap out of it and then send it back once I figure out how much power it will take to destroy it. See, this is why we can’t have nice things.
The SA SX-RK3 I tested had the grip shifter which seemed to work fine, unless you are using a right-hand throttle. I was using a right-hand throttle so I swapped the grip shifter to the left-hand side and all the numbers were upside down. This was done intentionally to confuse potential bike thieves. There is also a trigger shifter which I have never used but many people have recommended that I try that is sold by SA and I’m told that it works properly with the Nexus-3 IGH as well. The SA uses an industry-standard 3-speed IGH cog and can take a cog down to 13T according to their spec sheet.
As you can see from the chart above when the SA IGH is in the center gear it is a 1:1 gearing ratio. This might have a lot to do with why the SA hubs are so indestructible, other IGH that have the LOWEST gear set to a 1:1 ratio tend to be more fragile because when you end up in the higher gears there are a lot more of them and they have to be smaller. Looking at the planetary gearing in the SA 3 speed IGH the gears are big and have lots of teeth. I intentionally tried to shift under load at 2000W (which you should never ever do) and it was silky smooth with no issues at all. The Nexus 3 in comparison make a lot of strange noises when you try to shift it under load that make it sound like if you keep doing it you will be sorry. There are lots of people on the internet that claim to have run 4000W through this hub for many miles with not issues, but they never shifted under load.
The only complaint I had about the SA 3 speed IGH was the continuous clicking from the freewheel when the bike was being pedaled normally. Although everyone who rides a bike (except those hipster fixies) is used to the sound of clicking free wheels, but not when we are pedaling down the road, only when coasting. I have to admit that the constant clicking noise that emanated from the SA hub was pretty annoying. I’ve heard that this clicking goes away after a while, but mine hasn’t yet. Although it’s not that audible, it is constant, and it is annoying (kind of like my wife’s cat).
Figuring out the right gearing for the SA is tricky. Your best bet is to get the cog that you think you are going to need then ride the bike around and see if you need a bigger or smaller chain wheel or cog. The cogs for the 3-speed IGH are really cheap (I got my 15T for about $6 shipped on ebay) so it’s usually easier and cheaper to swap the cogs than the primary chain wheel. For most builds the rule of thumb is that if you want to use if for high-speed road riding you want a much smaller cog in the rear (around a 14T or so). If you are using it for trail riding then you will want to buy the biggest cog you can get for this IGH which is a 22T. There is a guy who runs the Bikesmith who sells aftermarket 26T cogs for a whopping $27 located here which would probably work best if you were running a 42T chainring (like the Lekkie or the Luna Eclipse rings) and wanted to have any prayer of having decent torque when trail riding. Adding a larger cog will cut into your top speed, but will give you way more power.
In short, the Sturmey Archer 3 speed IGH seems to be a realistic, viable option for 170mm rear dropouts that will work with most mid drives up to 3000W without issues. If you can get past the clicking and the cost and are willing to lace up your own wheel then it might be a great option for you. The SA 3 speed IGH is only available with 36 spokes and almost every fat bike rim in existence is 32 Hole. The only 36H fat bike rim I have found that will work with this IGH is the Weinman HL80 (available here) which is quite a heavy rim without punchouts and runs for around $50 online. The SA 3 speed IGH will only work with dropouts that accept threaded or quick release axles and will NOT work with through hole axles. Be aware that with a single speed IGH you will want either horizontal dropouts or a chain tensioner as otherwise it will be impossible to get the chain tension right. A loose chain at high power makes for all kinds of problems.
Destroy all monsters.